1. Field of the Invention
The present invention relates to an optical submarine communication system, and more particularly to an optical submarine communication system comprising a terminal apparatus installed on land for transmitting an optical signal and electric power, a land cable connected to the terminal apparatus, an optical submarine cable connected to the land cable and including an optical fiber and a feeder line for respectively transmitting the optical signal and the electric power from the terminal apparatus, and a repeater connected to the optical submarine cable, driven with the transmitted electric power and amplifying the optical signal, wherein a surge generated from the land cable side because of a lightning stroke or an insulation failure, is suppressed to prevent damaging the repeater.
2. Description of the Related Art
FIG. 19 is an explanatory view showing a schematic arrangement of a conventional optical submarine communication system using an optical submarine cable, and FIG. 20 is an explanatory view showing a manner in which a land terminal station, a land cable and an optical submarine cable are connected in the conventional optical submarine communication system. Referring to FIGS. 19 and 20, numeral 1 denotes a terminal (station) apparatus installed near the seashore of each of lands, e.g., lands A, B and C for transmitting an optical signal and electric power, and 2 denotes a land cable connected to the terminal apparatus 1. Numeral 3 denotes an optical submarine cable connected to the land cable 2 through a beach manhole 4 and including an optical fiber and a feeder line for respectively transmitting the optical signal and the electric power from the terminal apparatus 1. Numeral 5 denotes a repeater that is installed on the sea bottom, is connected to the optical submarine cable 3, and amplifies the optical signal. Numeral 6 denotes a branch unit connected to the optical submarine cable 3 and branching the optical signal.
The terminal apparatus 1 is provided with a surge protective device (not shown) and cable grounding to earth at the terminal apparatus 1 itself. Such an arrangement protects internal units of the terminal apparatus 1 against a surge caused when an induction current is generated due to a lightning stroke on a cable and enters the cable. Also, with the above arrangement, a surge generated due to a lightning stroke just on the terminal apparatus 1 or thereabout or with disconnection or an equipment failure occurred in the terminal apparatus 1, is suppressed from entering the land cable 2.
In such a conventional optical submarine communication system, the distance from the seashore to the terminal apparatus 1 is relatively short (about 2 km) and therefore the length of the land cable 2 is also short. As a recent tendency, however, a longer-distance optical submarine communication system is installed and the length of a land cable is increased (over 20 km) as practiced, for example, in the Denmark station of the optical submarine cable system TAT-14 installed by KDDI-SCS. The longer distance of the land cable 2 increases the area in which the land cable 2 may suffer from the adverse effect of a lightning stroke. This results in more frequent surge entering the land cable 2.
Further, when the land cable 2 is relatively short, a surge protective device (not shown) provided in the terminal apparatus 1 and cable grounding to earth at the terminal apparatus 1 itself are also effective in suppressing a surge occurring in the land cable 2. With an increased distance of the land cable 2, however, it is inevitable that some part of the land cable 2 exists outside the range over which a surge is suppressive. This raises a problem in that when a surge caused on the long-distance land cable 2 because of a lightning stroke or disconnection, is going to reach the repeater 5, the surge cannot be suppressed by the surge protective device and the cable grounding at the terminal apparatus 1.
Moreover, it has been thought in the past that grounding the land cable 2 at a portion thereof is effective in suppressing a surge and the object intended by the grounding is sufficiently fulfilled. Such grounding is surely effective in suppressing a surge, but the grounding point also becomes a point through which a surge enters. The reason is that a lightning stroke on the land cable near the grounding point causes a lightning current to flow into the land cable through the grounding point and increases a grounding potential. The increased grounding potential gives rise to a surge voltage in the land cable. Accordingly, this raises a problem in that a satisfactory surge suppressing means cannot be obtained simply by grounding the land cable.
Usually, a surge protective device (not shown) is also provided in the repeater 5. However, since such a surge protective device is provided to prevent a surge from passing a path including the surge protective device and from reaching a main circuit of the repeater 5, it does not have the function of attenuating or suppressing the surge. This has raised a problem that a surge is shut off from one repeater, but is allowed to propagate toward another repeater. Although the surge is eventually attenuated with resistance of the cable, there is a possibility that a high-voltage surge may propagate until it is attenuated.
Thus, a problem has been experienced in that a surge generated in the long-distance land cable 2 because of a lightning stroke or an insulation failure, may propagate to the repeater 5 on the sea bottom through the feeder lines of the land cable 2 and the optical submarine cable 3, and may damage devices inside an amplifier provided in the repeater 5.